more_complex_fmapping.md

I actually used this in my treezip submission to get my zipping function into the tree. If you're looking for a more complicated example, fmap can let you apply functions in this sort of way also:

instance Functor Tree where
  fmap f Leaf = Leaf
  fmap f (Node elem left right) = Node newelem newleft newright
    where newelem  = f elem
          newleft  = fmap f left
          newright = fmap f right

That's not important, so leave it if it makes you uncomfortable.